Prime Minister David Cameron is expected to outline plans to lawmakers in parliament that would nearly half Britain's force in Afghanistan, which is currently around 9,000-strong.

A Downing Street source confirmed to AFP on condition of anonymity that an announcement would be made on a troop withdrawal on Wednesday but would not give details of numbers.

Britain has already said it intends to pull out all its combat troops by the end of 2014 and hand over security responsibility from the NATO-led International Security Assistance Force (ISAF) to Afghan forces.

Cameron discussed the plans with US President Barack Obama on Tuesday, Downing Street said in a statement, adding that they agreed the handover plan was "on track".

"This would present further opportunities for ISAF countries to bring troops home next year and they agreed to stay in close touch as detailed plans develop," the statement said.

"They also agreed on joint work to strengthen the political process, particularly supporting Afghanistan and her neighbours to work together for stability, building on the trilateral discussions with Pakistan led by the United Kingdom."

Cameron is expected to speak on the issue during prime minister's questions on Wednesday and Defence Secretary Philip Hammond is due to give a statement on Afghanistan afterwards.

British newspapers and broadcasters said around 4,000 troops would be pulled out over the course of 2013, but that levels were likely to be kept at around 5,000 until the handover at the end of the following year.